Department of Transportation statistics indicate that fires are one of the major causes of fatalities in automobile accidents. In turn, a major cause of such fires is shorted electrical circuits in the automobile, causing sparks which ignite gasoline vapors. A shorted automobile battery, as where sheet metal is forced against both battery terminals during a collision, is a frequent cause of such fires, because the high current available from such devices facilitates sparking. Thus, a need has existed for a device suitable for prevention of battery shorts. Further, since gasoline vapors may permeate a vehicle following a collision or other accident, a device suitable for preventing electrical shorts elsewhere in the vehicle has been needed.
One means, well known in the art, for disconnecting an electrical circuit is a mercury tip switch, which typically comprises a reservoir of mercury disposed between two electrical contacts. When the circuit is to be completed--i.e., the switch closed -- the switch is tipped so that the mercury strikes both contacts simultaneously. When the switch is to be opened, the switch is tipped so that a gap exists between the mercury and at least one of the contacts. The difficulty with such tip switches in preventing sparking during automotive impact or roll over is that the switch may open for a moment, but then return to the closed condition. Such a return to the closed condition would render the switch valueless since electrical sparking, and therefore fires, could be started when the switch returned to the closed position. Also, such tip switches typically have not been capable of carrying the currents required of a car battery during starting and other operations.
Thus, a need has existd for a tip switch capable of carrying the currents supplied by a car battery and remaining in the open position upon impact or roll over until reset.